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By Tim Blythman Remote Controller DCC Booster Stepper Motor Driver μDCC Decoder Stepper Motor Driver and Decoder Stepper motors are capable of remarkably precise movement but are more difficult to control than a brushed DC motor. This compact board drives stepper motors with ease and can be configured to work with different control inputs, including direct current and Digital Command Control (DCC). Image source: https://unsplash.com/photos/miniature-train-set-with-detailed-landscape-rNOwodoejTc Y ou might have seen that tiny stepper motors are available quite cheaply from online marketplaces like eBay and AliExpress. A typical example is the assembly used to move the laser head on a DVD drive (see the photo below). This one is fitted with a helical shaft for linear control of the head assembly position. I have been curious about whether it would be possible to use such a motor to drive a model locomotive. Tiny DC motors are available, but are known for high speed and low torque, which is not a good match for the wheels on a model locomotive. Compact coreless DC motors are often used in This stepper motor is 15mm in diameter and works well with our Driver. The assembly is similar to the type used in CD/DVD/Blu-ray drives to position the laser head. 50 Silicon Chip quadcopters, where they only operate at high speed. Adding a gearbox can provide appropriate speed and torque, but also adds complexity and uses up valuable space. Many stepper motors can be driven slowly and still provide useful torque directly from their output shaft. Stepper motors require different control circuitry; they typically have two or more windings that are energised in sequence to control the speed and direction of the motor. It’s not possible to apply a DC voltage as can be done for a simple brushed motor. In this context, DC means a relatively steady voltage of either polarity, or perhaps a PWM (pulse-width modulated) voltage. We’ve published articles about stepper motors and the hardware needed to drive them by Jim Rowe in the past. The Quick Primer on Stepper Motors (January 2019; siliconchip.au/ Article/11370) is a good place to start if you aren’t familiar with how stepper motors work. He also wrote about some stepper motor driver modules in Part 22 of the Cheap Modules series (February 2019; siliconchip.au/Article/11405). This type of module makes it easy to control a stepper motor using a microcontroller, but I thought it would be handy to drive a stepper by applying a DC voltage in the same way you might power a brushed motor. I realised that our recent DCC Decoder (December 2025; siliconchip. au/Article/19377) already has most of This tiny board (shown at actual size in the lead image and on the right) can run a small stepper motor as though it were a DC motor. It can also operate in DCC mode (with some parts left off), using a stepper motor to power a model locomotive. Some of the stepper motors and assemblies that we were able to control with the Driver are also shown in the lead; the largest is 15mm in diameter. The smaller two motors are only about 5mm in diameter and ran quite hot, so we recommend using a lower current limit if driving such motors. siliconchip.com.au Features & Specifications 🛤 Four motor connections to suit bipolar stepper motors 🛤 Two additional current-limited opendrain Mosfet outputs 🛤 Adjustable speed response 🛤 Selectable drive current limit 🛤 Can be configured for DC or DCC operation 🛤 Maximum peak input voltage: 17V 🛤 Motor drive current: up to 500mA 🛤 Accessory outputs: up to 100mA 🛤 Module size: 24 × 13 × 4mm the components needed for driving a stepper motor. The firmware that provides the DCC decoding function would just need to be adapted to drive a stepper motor instead of a DC motor. So this Stepper Motor Driver and Decoder (we’ll call it the Driver for short) has two operating modes. It can accept a DC voltage and generate a waveform to drive a stepper motor as if it were a DC motor. In other words, the polarity of the applied voltage determines the direction of rotation, and the magnitude determines the speed. The other mode is to behave as a DCC decoder. Instead of a brushed DC motor, it has outputs that can be used to drive a small stepper motor. Since it has much in common with the earlier Decoder design, we recommend that you read the DCC Decoder article if you have not already done so. The Driver also has two open-drain outputs that can sink current. In DCC mode, these work in the usual fashion as DCC function outputs (for lights or similar accessories). In DC mode, one switches on for one input polarity and the other for the reverse polarity, providing a similar directional lighting function. DCC PROJECT KITS DCC Decoder, December 2025 (SC7524, $25) includes everything in the parts list DCC Base Station, January 2026 (SC7539, $90) includes everything in the parts list, except for the case, power supply, glue and the CON4 & CON5 headers DCC Remote Controller, February 2026 (SC7552, $35) includes all required parts, except for the UB5 case and wire/cable DCC Booster, March 2026 (SC7579, $45) includes all required parts, except for the Jiffy box, OLED screen, power supply and front panel. The OLED screen (SC7484, $7.50) and front panel (SC7578, $5.00) are available separately. DCC Stepper Motor Driver & Decoder, April 2026 (SC7601, $30) includes all required parts for DC or DCC mode. A with a positive voltage, then coil B with a positive voltage. The next phase is to drive coil A with a negative voltage, followed by coil B with a negative voltage. The cycle then repeats. These four phases correspond to the four steps of the stepper motor’s rotation. To drive the motor in reverse, the sequence is reversed. Note that reversing the polarity of one coil will have the same effect as reversing the sequence. Most stepper motor drivers employ micro-stepping, which effectively interpolates the output between each phase to create more, smaller steps. Our Driver implements 256 microsteps, where the four phases noted above correspond to microsteps 0, 64, 128 and 192. We use PWM (pulsewidth modulation) to interpolate between the steps. Scope 1 (with filtering applied for clarity) shows the voltage at the four stepper motor connections as the Driver progresses through its cycle. A+ 6V B+ A− For example, microstep 32 (between microstep 0 and microstep 64) drives both coils A & B with a 50% duty cycle in the positive direction. Internally, the microcontroller has a counter that dictates the current microstep. Having 256 microsteps means it is simple to loop around when the counter overflows; we can just ignore all but the lowest eight bits of the counter. The counter is incremented every 200μs (at 5kHz) and the increment determines the rate at which the cycle advances and thus how fast the motor turns. Applying a negative increment reverses the cycle and thus the direction of the motor. It isn’t expected that the motor will be precisely positioned to within 1/64th of a step, but the choice of that many divisions allows the speed to be set with a reasonable resolution while keeping the arithmetic simple for the 8-bit processor. B− A+ 4V Driving a stepper motor The distinguishing feature of the Stepper Motor Driver and Decoder is that it can drive a stepper motor, so let’s look at how that works in the firmware. The two H-bridge outputs are intended to connect to the two coils of the stepper motor, which are often denoted as A and B. The outputs are driven in a specific sequence to rotate the motor’s shaft. A typical waveform might drive coil siliconchip.com.au 2V 0V Microstep 0 Microstep 64 Microstep 128 Microstep 192 -2V 0.0ms 20.0ms 40.0ms 60.0ms 80.0ms Scope 1: this shows a typical waveform for driving a stepper motor from the unit. The shape of the waveforms means that the power draw is quite steady, regardless of the current motor drive phase. Australia's electronics magazine April 2026  51 The PWM is applied in a complementary fashion, so that at any instant, exactly one output is being driven, which keeps the load relatively constant. This also ensures that the current limit is applied uniformly at all times. Circuit details Fig.1 shows the circuit diagram for the Stepper Motor Driver and Decoder; it has a striking similarity to the DCC Decoder noted earlier. The main difference is that this circuit boasts two motor driver ICs to provide the fourwire connection needed by bipolar stepper motors. Since it is so similar to the Decoder, we’ll focus mainly on the differences. The incoming power supply connects to bridge rectifier BR1, which provides a voltage that we’ve labelled as a nominal 12V. In practice, the incoming supply can vary from 0V up to around 17V. The 17V limit is set by REG1’s maximum input voltage of 16V (allowing for a 1V drop across the bridge). REG1 provides a 3.3V rail – both rails have 10μF bypass capacitors. IC1 is a 14-pin, 8-bit PIC16F18124 (or -5 or -6) microcontroller that is powered from 3.3V, while IC2 and IC3 are DRV8231 motor driver ICs that are powered from the 12V rail. Their four outputs (available as motor connections A and B) require four control signals from the microcontroller. The keep-alive circuitry comprises diode D1, a 100W resistor and an optional capacitor. IC1 has a 100nF bypass capacitor and a 10kW resistor on its MCLR pin. This is exactly the same as the corresponding circuitry on the DCC Decoder. Similarly, the ICSP connections to pins 1, 4, 12, 13 and 14 allow IC1 to be programmed if needed. The two Mosfets, Q1 & Q2, are driven from a further two digital outputs of IC1. In the Decoder article, we described the current-limiting circuitry on the Mosfets (enforced by the 100W source resistors) and how the 0.68W resistors on the ISEN pins of the DRV8231 ICs set a 500mA limit on their outputs. The connections to sense the incoming voltage are different from the Decoder, since we need to measure the amplitude of that voltage. 100kW/10kW dividers bring both voltages down to a safe range for IC1’s ADC (analog-todigital converter) to measure, while the 10μF capacitor between the middle of the dividers low-pass filters the signal. This filter means that it is possible to apply a PWM drive signal to the inputs, and the filter will provide the average input voltage to the microcontroller for measurement. This chip has a differential ADC, so we can directly measure the difference between the two voltages, giving us the polarity and amplitude of the applied voltage. When configured for DCC operation, the low-side resistors and the 10μF capacitor are left off, providing the same sensing configuration as used in the Decoder circuit. The 10kW/10kW divider across the +3.3V rail, connected to pin 8 of IC1 (CONFIG) is used to configure the adjustable speed response, as mentioned earlier. Using different values here will provide different speed responses. Leaving the upper resistor off will force CONFIG to 0V, and the Stepper Motor Driver and Decoder will then operate as a DCC decoder instead of responding to the applied DC voltage. Firmware In DC mode, we can expect a varying input (supply) voltage. Below 5V Fig.1: the circuit of the Driver is similar to that of the DCC Decoder published previously. Since we need to control four motor outputs, there are only enough spare I/O pins to provide two open-drain function outputs. 52 Silicon Chip Australia's electronics magazine siliconchip.com.au at the bridge rectifier inputs, there is about 3.5V on the input to the regulator, and it is barely able to maintain its 3.3V output. When it starts up, the microcontroller samples the voltage on its pin 8 input (the “CONFIG” signal), to set its speed and mode. The microcontroller repeatedly measures the input voltage, but does not drive any of the outputs. With less than 4.5V on their supply pins, IC2 and IC3 stay in under-voltage lockout. Above 5V at the input, the micro enables Q1 or Q2, depending on the supply polarity. In a model railway application, these would be used to drive directional lights, with current sourced from the 12V rail. At 6V, IC2 and IC3 are now receiving 4.5V and will enable their outputs if commanded to do so. The micro subtracts 6V from the input voltage and uses that value, combined with the speed setting and input polarity, to generate an output waveform to drive the stepper motor. The 6V offset means that it is possible to easily achieve low-speed control. It also means that, for example, applying 12V to the input will drive the motor at double the speed compared to a 9V input. The analog reading of the CONFIG input is transformed into a ratio that reflects the ratio of the resistors used to set the voltage. Using ratios makes it easier to calculate the values needed to achieve a specific speed. For example, if the default 10kW: 10kW divider gives a certain speed, then changing it to a 20kW:10kW divider will give double that. The upper ratio limit is 10:1 (ten times the default speed), while the lower limit is about 1:100 (1% of the default speed). If the upper resistor is a very high value or left off (giving a lower ratio than 1:100), the Driver starts up in DCC mode instead. In DCC mode, the Driver operates like a DCC Decoder described in the earlier article, except that its motor outputs are arranged to drive a stepper motor. Since a DCC Decoder has configuration variables (CVs) for setting the speed response, we don’t need the CONFIG divider for this purpose. Notes In DC mode, the default speed (with the 10kW:10kW divider fitted) at 7V (1V above the 6V threshold) is 30 steps per second, or 7.5 full cycles of the output siliconchip.com.au waveform per second. This becomes 300 steps per second at 16V (10V above the 6V threshold). Of course, you can change this by changing the divider. In DCC mode, CV5 is used to set the speed ratio. CV2 and CV6 are not implemented, since the low-speed behaviour of stepper motors does not require the compensation that these CVs offer. The default value of CV5 is 64, implying that the speed can be increased approximately fourfold by setting CV5 to 255. A value of 64 for CV5 corresponds to 155 steps per second at speed step 127. This also means that a value of 52 for CV5 gives 127 steps per second at speed step 127; this value might be easier to use as a base for calculations. There is a very wide variety of stepper motors available, with different numbers of steps per revolution, winding resistances, output torque and shaft arrangements. So we recommend doing your research before connecting a stepper motor to ensure it works as best it can and doesn’t burn out or get damaged. We tested various stepper motors, ranging from a tiny unit measuring just 4mm across up to a so-called NEMA-8 unit. The NEMA-8 stepper motor looks similar to the NEMA-17 motors used in 3D printers, but is about half the diameter. The smallest motors worked well enough but got quite hot. So we recommend changing the 0.68W resistors to a higher value to reduce the current limit with such motors. The current limiting is based on a 0.33V threshold, so the formula is 0.33V ÷ I, where I is the target current in amps. For example, a limit of 100mA (0.1A) would require the 0.68W resistors to be replaced with 3.3W resistors. Don’t go any lower than 0.68W for the current sensing resistors, since that could result in the bridge rectifier exceeding its 1A limit. The NEMA-8 motor struggled to generate torque and would stall easily. These have a very low winding resistance and normally operate with a much higher current than the 500mA that is available from the Driver. Motors in between these sizes, around 8mm to 15mm in diameter, seemed to work quite well and typically had coil resistances of around 20-30W. The motor shown at the bottom of page 50 has a 15mm diameter. Motors with gearboxes will generally Australia's electronics magazine provide more torque; we found that even the cheap and common 28BYJ-48 type motors worked well. These have five wires, since they are arranged for unipolar operation, but if the common (typically red) wire is left disconnected, the Driver can power them. There are many varieties of stepper motor around, and we cannot characterise all of them. Still, the above should give you some idea about what motors will work best and how to adjust the Driver for the best operation. You should also check the stepping rate that your motor can support and ensure that you are operating within that range. Operating above the maximum stepping rate will cause the stepper motor to lose torque and possibly stall. Like the Decoder, connections to the board are made by soldering directly to surface-mount (holeless) pads. During our testing, we soldered headers to the various pads to easily try out different connections. Most connections are close to 0.1in pitch (2.54mm), so standard headers and sockets should work if you want to experiment. The firmware has been compiled to fit in the smaller flash memory of the PIC16F18124 (7kiB), but since the register maps are the same, the firmware should work without issue on the PIC16F18125 (14kiB) and PIC16F18126 (28kiB). Construction Being effectively set by a resistor, the firmware operating mode is fixed once construction is complete. To set the Driver to work in DC mode, all the parts listed should be fitted to the PCB. This is seen in the overlay diagrams, Figs.2 & 3. The resistors in the green The DCC version of the Driver leaves off three resistors and one capacitor to allow the firmware to switch to DCC mode and to properly sense the incoming DCC signal. April 2026  53 box are the CONFIG divider that can be used to alter the motor speed. To work in DCC mode, the high-side resistor (10kW) of the CONFIG divider is left off, as are the lower (10kW) resistors of the sense divider and the 10μF capacitor that provides filtering on the sense lines. These are labelled in red in Figs.2 & 3. If these were left on, they would interfere with sensing the DCC signal. We’ll describe fitting all the parts, so be mindful of which parts to leave off, depending on your intended use. Like other SMD projects, we recommend that you have flux paste, solder wicking braid, a magnifier, tweezers and some sort of fume extraction on hand. Working outside can help with avoiding smoke and fumes if you don’t have an extraction fan. Start assembly with the side shown in Fig.2, including REG1. Apply flux to all the pads on that side. Start with REG1, being careful not to mix it up with the similarly packaged Q1 and Q2. Rest it in place with the tweezers, tack one lead and adjust as needed. Then, solder the remaining leads. Install the two 0.68W resistors (or your other chosen value). These will be tricky to get to if they are soldered after IC2 and IC3. Next, fit the bridge rectifier, observing its polarity, and follow with the two 10μF capacitors on this side. Next solder IC2 and IC3, being sure to locate their pins 1 correctly. When the board is orientated as in Fig.2, the chip markings are upright, with the pin 1 dot at lower left. Then solder D1, making sure that its cathode stripe faces to the left, towards the bridge rectifier. Complete this side with the three resistors near D1. Note that it is only the 10kW resistor below the SC marking that is omitted for DCC operation on this side. Turn the board over and apply flux to the pads on this side of the board. Following Fig.3, solder IC1, noting that its pin 1 dot is at top right. Follow with the two SOT-23 transistors at upper right. That just leaves the passives. The CONFIG resistors are at upper left on this side, with the high-side resistor in the divider being the one closest to IC1; this is left off for DCC operation. The other two parts to be left off for DCC are at lower left, below IC1. Take care with the values of the remaining passives and note that the sole 100nF capacitor sits to the right of IC1. Clean the board using an appropriate flux cleaner and allow the board to dry. Inspect it for bridges and poor solder joints; repair any before attempting to power up the Driver. Testing and programming A weak power source, such as a 9V battery or current-limited supply set to around 12V and 100mA, can be applied to the T connections shown in Fig.2. You should see regulated 3.3V (3.2V to 3.4V) between the 3.3V and GND pins on the ICSP header. If that Figs.2 & 3: pay close attention to the components in the overlay diagrams and be sure to leave off the components marked in red if you are building the version for DCC. At a minimum, you should make connections to the T, A and B pad pairs to drive a motor; the other connections are not mandatory. 54 Silicon Chip Australia's electronics magazine The DC version is populated with all the components. The CONFIG divider has been set to its default of two 10kW resistors; these values can be changed to alter the speed response of the Driver. siliconchip.com.au is not right, or your power supply goes into current limiting, check your construction again. Further testing will require the chip to be programmed. If you have bought a programmed chip or kit from the Silicon Chip Shop, then this should not be required. Otherwise, solder a fiveway pin header to the ICSP header and connect it to a programmer such as a Snap, PICkit 4, PICkit 5 or PICkit Basic. The power supply noted above should be adequate if your programmer cannot supply power. Program the 0911124S.HEX file and check that the programming and verification complete successfully. Connections Figs.2 & 3 also show the connections that can be made. Note that the two pads marked A must connect to opposite ends of the same winding on the stepper motor, while the pads marked B connect to the two ends of the other winding. The easiest way to check the windings is to test for continuity, although this may not apply to five-wire motors. Just like the DCC Decoder, you can connect a capacitor to the + and – keep-alive connections to store and later provide energy if the supply is intermittent. While this was intended to handle dirty tracks in a model railway, it can also be helpful if you are trying to power the Driver with a PWM power source. The pad marked 12V is simply rectified DC from the bridge, so could vary over a wide range, especially if the Driver is being used in the DC configuration. If you want to use the Q1 and Q2 outputs with a varying supply, you Table 1 – supported configuration variables CV# Notes Default value 1 7-bit short address 3 3 Acceleration rate 0 4 Deceleration rate 0 5 Speed scaling: the default value of 64 results in 155 steps per second at speed step 127. Other values scale proportionally; for example, a value of 52 gives 127 steps per second at speed step 127. 64 7 Manufacturer version number (read-only) 0x5D 8 Manufacturer identification number (read-only) 13 11 Packet timeout 0 17 Most significant bits of long address 192 18 Least significant bits of long address 0 19 Consist address and direction 0 29 Configuration 2 33 Function mapping 1 34 Function mapping 2 35 Function mapping 0 36 Function mapping 0 37 Function mapping 0 49 Function effect bitmap for forward light output 255 50 Function effect bitmap for reverse light output 255 could use a constant current-circuit instead of the resistors shown in Fig.3. An alternative would be to feed the LEDs from 3.3V on the ICSP header, although this will offer much less headroom. You shouldn’t draw more than about 10mA from the 3.3V supply due to dissipation in REG1. DC use If you have a stepper motor connected, you can test out the Driver by applying a voltage at the T input to the bridge rectifier. We used a 9V battery for much of our testing; it went flat fairly quickly, but it was able to rotate Parts List – DC/DCC Stepper Motor Driver 1 double-sided 13 × 24mm PCB coded 09111242, 0.8mm thick 1 PIC16F18124-I/SL (or 18125 or 18126) 8-bit microcontroller programmed with 0911124S.HEX, SOIC-14 (IC1) 2 DRV8231DDAR motor driver ICs, SOIC-8 (IC2, IC3) 1 MCP1703A-3302E/CB 3.3V low-dropout linear regulator, SOT-23 (REG1) 2 2N7002 SOT-23 N-channel Mosfets (Q1, Q2) 1 1A SMD bridge rectifier (BR1) [MBS4 or CD-MMBL110S] 1 1N5819WS SOD-323 schottky diode (D1) 1 3cm length of 20mm diam. heatshrink tubing (to protect & insulate Driver) Capacitors (all SMD M2012/0805 size MLCC) 3 2 10μF 25V X5R 1 100nF 50V X7R Resistors (all SMD 1%, M2012/0805 size, ⅛W unless noted) 2 100kW 7 4 10kW 1 100W 2 10W 2 0.68W ¼W n values are to suit DCC mode. The values of two of the 10kW resistors can also be adjusted to change the speed response in DC mode. siliconchip.com.au Australia's electronics magazine all the stepper motors that we tested. DCC operation You will need a DCC signal to test out the Driver in DCC mode. The earlier parts of this series (siliconchip.au/ Series/455) describe a few options for Base Station Hardware. Table 1 lists the configuration variables (CVs) that are implemented on the Driver when it is operating in DCC mode. Apart from CV5, the other CVs will work in much the same fashion as those described in the Decoder article (December 2025). Thus, the remaining CVs have only brief descriptions of their characteristics. Note that we have used a different version ID (CV7) so that you can tell these Decoders apart. Otherwise, the DCC code is much the same, and the Driver should operate much like the earlier Decoder in all other respects. Conclusion We don’t expect that all stepper motor types will work well with this Driver. It is something of an experimental device; it originally began as a DCC Decoder that could be used to drive stepper motors. Still, we think that the ability to accept a DC voltage for power and control will be adopted for cases where basic operation of a SC stepper motor is needed. April 2026  55